Pellet mills include ring extrusion presses and flat die extrusion presses. Ring extrusion presses generally comprise an annular die having an inner surface facing the center point of the annular die and an outer surface facing away from the center point. The annular die thus resembles a finger ring, i.e. a round band. Die holes, typically radially directed, extend from the inner surface to the outer surface to allow the material to be pelletized to pass through the holes from the inner surface of the die to the outer surface, thereby forming strands of the material extending radially out from the outer surface. One or more knives arranged to move in relation to the annular die then cuts off the strands to form pellets. The material is supplied to the inner surface of the annular die whereupon it forms a feed layer. Rollers rolling over the inner surface of the annular die press the feed layer towards the inner surface of the annular die with high pressures, thus forcing the material in the feed layer through the die holes.
Flat die extrusion presses comprise a flat circular die, typically held stationary in a horizontal orientation, with the rollers being carried on axles extending radially from a vertically oriented rotating axle to roll over the die in a carousel fashion.
Pelletization is especially interesting for bio-based applications because the process allows the compaction, and therefore the increase in specific density, i.e. increased energy per bulk volume e.g. MJ/kg, of lignocellulosic based feedstock such as wood chips, saw dust, straw, and other biomass sources and also forest-based or agro-based waste materials, or feedstocks for feed pellets. This biobased feedstock can also have been pretreated in different ways (e.g. maturated, shredded, chipped, milled, etc.) and in different processes (e.g. thermally treatments like torrefaction, pyrolysis, etc.). In addition to increasing the density of the material the formed pellets are hard and thus easy to handle.
Pelletizing biomasses, and indeed any type of material to be pelletized, however requires that the material has the proper characteristics, in particular particle size and moisture content, if a useable feed-layer is to form. Here a usable feed layer is one which adheres sufficiently tight to the die while at the same time provides enough friction that the rollers, which are typically free rotating, i.e. not driven separately, will be caused to roll as the annular die rotates, or vice versa. Material to be pelletized is therefore typically ground or milled to a desired particle size, prior to or together with being moistened, or dried, to obtain the correct moisture content, before the now preconditioned material is supplied into the pellet mill.
Some materials, when preconditioned as needed for the pelletization process, however obtain characteristics which render the transport of the material into the pellet mill, or even the handling of the material during the preconditioning, difficult. Materials may for example become sticky, for example due to having low equilibrium moisture contents, so as to foul the equipment used for the preconditioning and bridging in conveying equipment. Further the sticky material becomes very difficult to transport e.g. in augers. These characteristics may be inherent to the material, or the material may obtain these characteristics due to pre-processing steps such as torrefaction (carbonization) in the case of biomass such as wood. Other materials may be considered unsuitable for pelletization altogether as they may chemically react with moisture, thus precluding the mixing of these materials.
U.S. Pat. No. 2,887,718 describes an attempt to postpone the addition of moisture to the material by providing a pellet mill having an annular die with rollers, the pellet mill further comprising a conduit for directing a fluid such as water towards a spray head for spraying the fluid onto the inner surface of the annular die for moistening the material to be pelletized.
GB850046A further proposes a pellet mill in which the need for premixing of material to be pelletized is obviated by providing a port adjacent the annular die where a liquid can be delivered into the die.
In view of these attempts the present inventors have realized that while addition of moisture into the annular die may to some degree appear to solve the problem of handling difficult to pelletize materials, these attempts do not provide for fine control of the moisture content of the feed layer and the resulting pellets. Further, with rising die temperatures a large amount of the added moisture will evaporate so that the moisture content in the produced pellets is by no means guaranteed.
In addition these attempts do not provide for the realization of more advanced pellets.